**Core Concept**
Amyloidosis is a group of diseases characterized by the accumulation of amyloid, a fibrillar protein, in various organs and tissues. The diagnosis of amyloidosis involves identifying the presence of amyloid deposits in affected tissues, which can be achieved through various laboratory tests.

**Why the Correct Answer is Right**
The confirmatory test for amyloidosis is the biopsy of an affected organ, followed by histopathological examination to visualize amyloid deposits under a microscope. This is considered the gold standard for diagnosing amyloidosis. The biopsy sample is typically stained with Congo red dye, which binds to amyloid fibrils and exhibits green birefringence under polarized light. This characteristic feature is diagnostic of amyloidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum protein electrophoresis (SPEP) is a useful screening test for amyloidosis, but it is not confirmatory. SPEP detects abnormal proteins in the serum, which may indicate the presence of amyloidosis, but it does not directly confirm the diagnosis.
**Option B:** Immunofixation electrophoresis (IFE) is another useful screening test, but like SPEP, it is not confirmatory. IFE can identify specific types of abnormal proteins, such as monoclonal immunoglobulins, which are often associated with amyloidosis.
**Option C:** Urine protein electrophoresis (UPEP) is a useful screening test, but it is not confirmatory. UPEP detects abnormal proteins in the urine, which may indicate kidney involvement in amyloidosis, but it does not directly confirm the diagnosis.

**Clinical Pearl / High-Yield Fact**
The biopsy of an affected organ, such as the kidney, liver, or rectum, is the most reliable method for diagnosing amyloidosis. A sample of tissue is obtained through a minor surgical procedure, and the presence of amyloid deposits is confirmed through histopathological examination.

**Correct Answer: C. Biopsy of an affected organ.**